Conventionally, electronic blood pressure meters which determined blood pressure value based on changes in pulse waveform employed two methods to determine the diastolic pressure value. One method was to use the cuff pressure value at the moment the pulse wave was first detected as the systolic pressure and to consider the cuff pressure at the moment the maximum pulse wave was detected to be the average blood pressure. These values for systolic pressure and average pressure were then employed in a specified algorithm to calculate the diastolic pressure value. The other method was to use a value relative to the amplitude of the systolic pulse wave. When the pulse wave amplitude is decreasing, one can detect the point at which the pulse amplitude comes closest to the threshold value, which is a given ratio (approximately 70%) of the systolic pulse wave amplitude. The cuff pressure at this moment will be the diastolic pressure value.
In the first method of determining the diastolic pressure described above (calculating diastolic pressure value from systolic and average pressure values), it is assumed that the average pressure value is between the systolic and diastolic pressure values, and also that the value will lie 1/3 of the distance between the diastolic and systolic pressure values. However, blood pressure waveforms vary from person to person and according to the state of health of any given person, and there are many cases in which the average pressure will be quite different from that predicted by this formula. Thus there is a danger that the use of this method will produce large errors in diastolic pressure values depending on the conditions of measurement.
The second method described above for determining the diastolic pressure value (considering the diastolic pressure value to be the cuff pressure value at 70% of the systolic waveform amplitude when the amplitude is decreasing) is simply based on statistical experience and has only a weak theoretical basis. Poor reliability of measurement accuracy can also be problematic using this method.